1. Field of the Invention
The present invention generally relates to recording apparatuses, such as inkjet printers, and computer-readable recording media storing recording programs.
2. Description of the Related Art
Typically, an inkjet recording apparatus includes a carriage on which a recording head is mounted. The recording head is configured to discharge droplets of ink onto a recording medium, such as a sheet of paper, as the carriage is moved in a main scan direction. The ink droplets attach onto the recording medium, thereby forming a line of an image thereon. The recording medium is then transported in a sub-scan direction perpendicular to the main-scan direction using a transport roller and the like, and another line of the image is formed. This process is repeated to form the desired image on the recording medium.
In order to increase the speed of such an image forming process, it is preferable to employ a recording head that is extended in the sub-scan direction so that a large number of dots can be printed simultaneously along the sub-scan direction. However, such a recording head having an extended length in the sub-scan direction requires increased cost and is also technically difficult to realize. One method attempts to overcome this problem by installing multiple recording heads in a staggered manner so as to form a recording head unit that is virtually extended in the sub-scan direction, thus increasing the number of dots that can be printed simultaneously in the sub-scan direction.
However, when the multiple recording heads are arranged in a staggered manner, the position of the individual recording heads may be shifted from their ideal positions according to design (“normal positions”) depending on the location of the recording heads (including mechanically accuracy), the environment in which the recording heads are used, and the period of use of the recording heads, for example. The shift in the position of the recording heads from the normal position may cause a printing position error, resulting in an image break, density irregularities, and so on. Thus, when multiple recording heads are disposed in a staggered manner along the main-scan direction, it is important to provide a mechanism for preventing a printing position error in the sub-scan direction caused by the recording heads.
Japanese Laid-Open Patent Application No. 2004-358759 discusses a technique for accurately correcting a print position error between dots printed at different times. This technique involves printing a reference pattern and a correction pattern using plural nozzles of a recording head. Each pattern consists of dots printed on a printing medium, such as a sheet of paper, at certain intervals in the sub-scan and the main scan directions so that one or more columns of the dot patterns are printed at equal intervals along the main-scan direction. The technique is described as being capable of accurately controlling a print position error based on the reference pattern and the correction pattern.
However, the above technique does not take into consideration how to detect a printing position error in the sub-scan direction that is caused when plural recording heads are arranged in a staggered manner.